Add group-class withdrawal deadline and kind-aware offering form

Group classes now carry an optional per-class withdrawal deadline. Up to
that day a student may withdraw themselves from the class; the withdrawal
frees the seat and voids any pending payment but never issues an account
credit. After the deadline self-withdrawal closes and a studio admin must
withdraw the student by hand (the admin path is never subject to the
deadline). A blank deadline keeps self-withdrawal open indefinitely.

Also make the Add/Edit Offering form show only the fields relevant to the
selected kind: group settings for group classes, weekly reservation for
private lessons. Progressive enhancement — without JS every field renders.

- New nullable us_offerings.withdrawal_deadline column; Offering model gains
  $withdrawalDeadline + isWithdrawalOpen().
- New student endpoint POST /enrollments/{id}/withdraw, gated by the deadline
  (403 withdrawal_closed), ownership-checked, idempotent.
- Front-end group-class page shows a Withdraw button while open.
- No USC_VERSION bump: 1.2.0 is unreleased and accumulates schema changes
  under its section, matching the scheduled-billing and credit features.

Tests: composer test (596), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/Unit/Offering/OfferingControllerTest.php

@@ -139,6 +139,43 @@ class OfferingControllerTest extends TestCase
         $this->render();
     }
 
+    public function testAddGroupClassStoresWithdrawalDeadline(): void
+    {
+        $_POST = [
+            'usc_action'          => 'add',
+            'title'               => 'Ballet Beginners',
+            'kind'                => Offering::KIND_GROUP_CLASS,
+            'term_start'          => '2026-09-08',
+            'withdrawal_deadline' => '2026-08-31',
+        ];
+
+        $this->repository->shouldReceive('insert')->once()->with(Mockery::on(
+            static fn (Offering $o) => '2026-08-31' === $o->withdrawalDeadline
+        ))->andReturn(1);
+        $this->repository->shouldReceive('findAll')->andReturn([]);
+        $this->reconciler->shouldReceive('reconcile')->once()->andReturn(['removed' => 0, 'conflicts' => []]);
+
+        $this->render();
+    }
+
+    public function testBlankWithdrawalDeadlineLeavesItNull(): void
+    {
+        $_POST = [
+            'usc_action' => 'add',
+            'title'      => 'Choir',
+            'kind'       => Offering::KIND_GROUP_CLASS,
+            'term_start' => '2026-09-08',
+        ];
+
+        $this->repository->shouldReceive('insert')->once()->with(Mockery::on(
+            static fn (Offering $o) => null === $o->withdrawalDeadline
+        ))->andReturn(1);
+        $this->repository->shouldReceive('findAll')->andReturn([]);
+        $this->reconciler->shouldReceive('reconcile')->once()->andReturn(['removed' => 0, 'conflicts' => []]);
+
+        $this->render();
+    }
+
     public function testGarbageClassTimeIsRejected(): void
     {
         $_POST = [